Sand layer is the collective name of loose aquifer such as fine sand, coarse sand, gravel and cobble, alluvial debris, etc. As long as the sand layer has a certain burial depth, it is often more water output per unit, lower drilling cost and less energy consumption for pumping compared with bedrock fracture water, which is very popular among the public. In the past, most of them were measured by resistivity method and selected high value points. Now find water personnel are more likely to use the sky electricity method ground water detector, sky electricity method ground water detector to find sand layer water most people hope to find high value, the effect is not good, so that the sand layer can not find. After our unresolved efforts in recent years, now finally have a more profound understanding. Now our thoughts are organized as follows.
1, the sand layer and clay layer are rich in gravity water, polarity are very strong, intends to put the two in the ground water detector sky electrogram intuitive distinction between which is the sand layer, which is the soil layer, is very difficult to do.
In the sediments of the ancient river channel, the sand layer is deposited during the flood season, and the clay soil is deposited during the non-flood season when the flow rate is extremely slow, and the two are generally layered with each other, with coarse and fine layers, and obvious rhythm.
For example, in the attached Figure 1, the drilling record is a good well when there are many sand layers from 45 to 74 meters, the following is mudstone, and the water output is 30 square meters. However, it is not clear where the sand layer is at all in the part where there is a sand layer, and it does not show high value reflection like the electric method logging. This is one of the general rules of the sand layer measured by the ground water detector, that is, the sand layer and the soil layer mostly show the same electrical layer, which can be collectively called the loose sediment of the fourth system.
2, loose sediments in the ground water detector astronomical map, no matter how coarse the sand layer, most of them are low value reflection.
In the ground water detector astrograph because the saturated water content of the sand and clay layer is close, and are very large, generally up to 30 ~ 40%, but the clay water degree is very small, the water can not be discharged. So in the area without salty water, above the bedrock, the soil and sand layers mostly behave as low value layers. In the hill area and denuded quasi-plain area, the sand layer of river terrace is generally deposited by palaeochannels with coarse grains. The bedrock on both sides of the palaeochannel is shallow. The tenement values of the sand layers (including the pebble layer) in the palaeochannels are likewise low.
In Figure 1, under the loose sediment is mudstone, which generally has a lower TEN value, and the value of the loose sediment in this case is much lower than the mudstone, and even has more 0 values.
For example, in Figure 2, there is a 4-meter pebble layer in the shallow layer of point 6, and the following is very soft rock, according to the drilling rig, "100 meters of rotten mud was hit below the pebble, and it did not penetrate, so it was angry and did not hit. On the original profile, it is true that the 100 meters above is dark blue, which means that the 100 meters of "mud" mentioned by the driller is true, but on the processing profile, the value of the pebble layer is smaller than the alleged "mud", which further confirms that the value of the loose sediment is indeed is generally small. This is another general rule of measuring sand layer by ground water detector, that is, the value of loose sediment is small and small, mostly close to 0 value.
3. The best well location in the palaeochannel can be determined by using the low value downward transmission law.
Low value downward transmission is a universal law in the astrometric profile. The smaller the value of the shallow surface layer, the more obvious it will be that low value downward transmission occurs at the measurement point. The center of the palaeochannel has the greatest thickness of loose sediments and the lowest values, and the result of this extra-low value layer value downcast will appear thicker and deeper in the deep blue zone on the profile. Using this principle, the well location of the shallow palaeochannel profile with the thickest values will be the center of the palaeochannel, and the corresponding sand layer will be thicker and more abundant, and the water volume will be larger. Below we illustrate with three typical cases.
Case 1: The 9.5 in Figure 3 is a 50 cm diameter cement pipe well drilled by Mr. Zou in Laizhou City on the left bank of the Wang River on alluvial terraces, with a depth of 38 meters and a sand layer of 20 to 23 meters and a water volume of 30 square meters. This area within 500 meters there are more than 20 wells, all have sand layer. 2000 drought other wells are very little water, only he this well water is still very good. Mr. Zou to find out the reason, he measured a sky electric profile, point distance of 5 meters, found that other measurement points shallow values are larger, only this well near the value of the smallest, indicating that the well is in the center of the ancient river channel, the sand layer deeper, thicker, and there is a low value of downward transmission phenomenon. Looking for sand layer water should find such a low value point to set the well.
Case 2: The attached Figure 4 is a sky electric profile measured at a cell on the right bank terrace of the Bailang River in Weifang City. Here is the width of several kilometers near the center of the ancient river channel of the Bailang River, the thickness of the fourth system overburden more than 40 meters, 30 meters below the multi-layer coarse sand and gravel, local pebbles, the water is very rich, descending to a depth of 5 meters of the single well out of water up to 50 to 100 square hours or more, is the best conditions for groundwater source heat pump has been built in Weifang City, can do the full amount of recharge. The following is the basalt with bad water quantity. However, due to the complexity of geological conditions, even the best conditions are not uniform, the thickness of the sand layer and the coarseness of the particles at each well site are not exactly the same, and the unit water output of the well varies by more than a factor of one. In order to study whether the size of the value measured by the single frequency method can reflect the pussy of the sand layer water, we used the depth of 30, 50, 80, 120 meters to measure, the measured value is 0.000, until the depth of 180 meters to adjust some of the measurement points is no longer 0 value, indicating that the value is very small, intended to use the value of the single frequency method size to find the center of the ancient river channel is not successful.
Finally we changed to use multi-channel meter measurement, retesting completely consistent, in the processing of the map reflects the best location of the line of measurement of the ancient river channel in 6, 7, 8, 9, 11 five points, and the ground source heat pump heat source well unit water output size of the law of change is consistent.
Case 3: Figure 5 attached known well at point 10, 24 to 26 meters is coarse sand layer, static water level of 18 meters, the water output of 15 square hours. This depth is also a low value layer on the profile. If it is thought that the sand layer is also a high-value layer on the ground water detector TEMP, it will be thought that the sand layer is not reflected and the TEMP ground water detector is not effective in finding the sand layer. In this paper, we know the opposite - the shallow loose sand layer of the fourth system is not a high-value layer on the ground water detector sky diagram, but a low-value layer, and we should look for the low-value point where the low-value downward transmission is more obvious to set the well. It is different from the resistivity method to find the sand layer is to find the high value, they are also inconsistent in the principle of physical exploration, the two are not comparable. Therefore, we believe that the best well location for this line should be at points 1 to 7 and the more leftward the sand layer, the better.
In addition, according to Hunan Mr. Zhang recalled that two years ago he measured water in the red layer area, found a low value anomaly in the denuded quasi-plain, the actual hit found to be a layer of coarse sand and gravel layer, the daily output of water up to 1000 square, he also believes that the ancient river channel should find low value anomaly.
4, should pay attention to several issues.
First, in areas with deep fresh water, due to the thickness of the strata, the sand layer has a certain degree of compression and density, porosity and water content becomes smaller, the sand layer has a weak high value performance is normal. Some of the loose strata in the northwest of China are very deep, two or three hundred meters less than rock, in which the sand layer (including gravel layer or pebble layer) may have occurred weakly cemented or compacted, the sand layer can also be shown as a relatively weak high quality layer, you can find the relative high value layer of the number of points set well (it should be noted that if the water is measured with a multi-channel instrument, be sure to exclude the false horizontal high value layer). If there is a high value fly line in the bedrock below, it will make the sand layer that shows as a weak high value be covered up, you should artificially change that high value fly line on the computer to a smaller size, and the sand layer will show up.
The second is to find the center of the ancient river channel, the measurement line should be as long as possible, it is best to reflect the edge of the ancient river channel, like the right side of the attached Figure 2, the edge of the ancient river channel is very clear, indicating that the pebble layer has been pointed out, so it will look more intuitive.
Thirdly, due to the low value downward transmission, the thickness of loose sediment is much larger than the actual one on the original profile, and the thickness of the low value layer in the processing profile should be used as the main reference.
Fourth, the measurement value reflecting the loose sediment is very small, measured with a single-channel instrument because the data changes at any time, the value is not stable enough, it is best to measure with a multi-channel instrument.
Fifth, adhere to the known to unknown. The shallow low value downward transmission of many maps, not all can be loose sediments with sand layers. First of all, we should analyze the topography and geomorphology to determine whether there is a possibility of forming a palaeochannel. It is best to have known wells as a reference.
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